JPH0533157A - Production of chromated zinc or zinc alloy plated steel sheet excellent in black rust resistance - Google Patents

Abstract

PURPOSE:To produce a chromated hot-dipped steel sheet excellent in black rust resistance and darkish discoloration resistance without performing any aftertreatment. CONSTITUTION:An aqueous solution prepared by further incorporating one or >=2 kinds selected from the group consisting of phosphoric acid, acetic acid, hydrofluoric acid, and salts thereof by the amounts, expressed in terms of phosphoric acid, acetic acid, and hydrofluoric acid, respectively, in the ranges where (phosphoric acid)/(total chromium content)(by weight), (acetic acid)/(total chromium content)(by weight), and (hydrofluoric acid)/(total chromium content) (by weight) are regulated to 0.05/1 to 2.0/1, 0.1/1 to 1.0/1, and 0.01/1 to 0.1/1, respectively, into an aqueous solution containing chromic acid and chromic acid reducing product where (trivalent chromium)/(sexivalent chromium) (by weight) is regulated to 1/4 to 1/1 by 2-20wt.% expressed in terms of metallic chromium content is applied to the surface of a zinc or zinc alloy plated steel sheet, followed by heating and drying at 150-300 deg.C. By this method, a chromate film in which (sexivalent chromium)/(total chromium content) (by weight) and the coating weight of chromium are regulated to <=0.20 and 10-150mg/m<2>, respectively, can be formed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a chromate-treated zinc- or zinc-based alloy-plated steel sheet, more specifically, hot-dip galvanizing (including alloying hot-dip galvanizing and hot-dip Zn-Al alloy plating) steel sheet causes black rust And a method for producing a chromate-treated plated steel sheet capable of suppressing blackening.

[0002]

2. Description of the Related Art Conventionally, chromate treatment is widely used as a rust preventive treatment for hot dip galvanized (including galvannealed galvanized and hot dip Zn-Al alloy plated) steel sheets. It is known that black rust and anomalies occur and the appearance is significantly impaired. Of these, the abnormalities that occur relatively frequently in hot dip galvanized and Zn-Al alloy plated steel sheets are C before chromate treatment.
o A study has been published that it can be prevented by spraying a salt solution onto a steel plate (CAMP-ISIJ Vol 1 (1988) p667). However, regarding the black rust (black dot rust) that occurs relatively frequently in the galvannealed steel sheet, neither the generation mechanism nor the prevention method is clear and there is no countermeasure.

[0003]

As a result of a detailed study of the black rust phenomenon, the present inventors have found that the black rust phenomenon causes Cr ⁶⁺ remaining in the chromate film to elute in the water condensed on the steel sheet. Since chromic acid was formed and the pH of dew condensation water was lowered, the surface of the steel plate was corroded in a pitting manner, and it was confirmed that it was formed.

Therefore, it has been clarified that reducing the amount of residual Cr ⁶⁺ in the chromate film is an effective measure for suppressing the black rust.

Conventionally, for chromate treatment of hot-dip galvanized steel sheet, for example, JP-A-52-68036 and JP-A-6-6836.
As disclosed in JP-A-3-103082, a temperature of about 100 ° C. is generally obtained after spray-ringer or dip-ringer treatment is applied to the steel sheet using a liquid containing chromic acid as a main component and other acid or salt added as an etching aid. Generally, the method of drying treatment is performed. Therefore, Cr ⁶⁺ remains in the formed chromate film,
In some cases, the Cr ⁶⁺ / total Cr amount (weight ratio) may be 0.2 or more, and according to the studies by the present inventors, it has been clarified that this is the main cause of black rust generation. .

Therefore, the present inventors have conducted various studies on various methods for reducing the residual Cr ^6+, and as a result, reduce Cr ⁶⁺ in the chromate treatment liquid, and phosphoric acid, acetic acid, hydrofluoric acid and / or these compounds. Chromate treatment is performed using a treatment solution containing salts, and after treatment, the residual Cr ⁶⁺ is converted to C by drying at a drying temperature of 150 ° C or higher and 300 ° C or lower.
The present invention has been completed by discovering that the amount of r ⁶⁺ / total Cr can be reduced to 0.2 or less, and thereby the generation of black rust can be suppressed.

The inventors of the present invention not only generate black rust,
We also confirmed that it is effective in suppressing blackening. The present inventors have found that in order to suppress the generation of black rust, the residual Cr in the chromate film is
It is effective to reduce ⁶⁺ , and the present invention has been achieved as a specific method.

Therefore, an object of the present invention is to provide a method for producing a chromate-treated zinc or zinc-based alloy plated steel sheet which is excellent in corrosion resistance as well as black rust resistance and blackening resistance.

[0009]

[Means for Solving the Problems] That is, according to the present invention, trivalent chrome /
Chromic acid having a hexavalent chromium (weight ratio) in the range of 1/4 to 1/1 and a chromate-reducing product as the amount of metallic chromium are 2
In an aqueous solution containing 20 to 20% by weight, one or more selected from phosphoric acid, acetic acid, hydrofluoric acid and salts thereof is converted into phosphoric acid, acetic acid and hydrofluoric acid, respectively.
Total chromium amount (weight ratio) 0.05 / 1 to 2.0 / 1, acetic acid / total chromium amount (weight ratio) 0.1 / 1 to 1.0 / 1,
Hydrofluoric acid / total chromium amount (weight ratio) 0.01 / 1 to 0.1 /
After applying an aqueous solution containing in the range of 1,
To form a chromate-treated film having a hexavalent chromium / total chromium amount (weight ratio) of 0.20 or less and a chromium deposition amount of 10 to 150 mg / m ² by heating and drying in the temperature range of ℃ to 300 ° C. It is intended to provide a method for producing a chromate-treated zinc- or zinc-based alloy-plated steel sheet having excellent black rust resistance. The present invention can be suitably applied to hot-dip galvanized steel sheets, alloyed hot-dip galvanized steel sheets, and hot-dip Zn-Al alloy-plated steel sheets.

The present invention will be described in more detail below.

The present invention relates to a method for producing a chromate-treated zinc- or zinc-based alloy-plated steel sheet, and a steel sheet to which the method of the present invention can be preferably applied is a zinc- or zinc-alloy-plated steel sheet. Examples of the hot-dip galvanized steel sheet and hot-dip Zn-Al alloy plated steel sheet.

With respect to chromic acid, an aqueous solution mainly containing chromic anhydride is usually used in order to ensure corrosion resistance in chromate treatment and to enable uniform coating of steel material, but to suppress the generation of black rust. must be 0.2 or less in Cr ⁶⁺ is Cr ⁶⁺ / total Cr content in the chromate coating after treatment in (weight ratio), Cr ⁶ in the processing liquid in order to such coating ^Since it is necessary to reduce ⁺ and secure Cr ³⁺ at a constant ratio, Cr ³⁺ / Cr ⁶⁺ (weight ratio)
Was set to 1/4 to 1/1.

That is, Cr ³⁺ / Cr ⁶⁺ (weight ratio) is 1
When it is less than / 4, it is difficult to suppress the generation of black rust because it is difficult to make Cr ⁶⁺ in the dried chromate film 20% or less with respect to the total chromium amount (hereinafter referred to as T-Cr). Become.

If the amount of Cr ³⁺ is too large and exceeds 1/1, a precipitate is likely to be formed in the chromate treatment liquid, which causes inconvenience in chromate treatment work and storage of the treatment liquid.

Next, phosphoric acid and phosphate will be described. Phosphate radicals have the effect of increasing the stability of Cr ³⁺ in the chromate treatment solution, and have an etching effect on the zinc plating of the material, so that they are suitable for relatively inert plating surfaces such as hot dip zinc. It has the effect of evenly attaching the chromate.

However, if it is less than 0.05 with respect to T-Cr, the effect is small, and if it exceeds 2.0, the etching property of the chromate solution is excessively increased, which tends to cause unevenness of treatment, so that 0.05 to 2 is caused. The range was limited to 0.0 (weight ratio).

Acetic acid and acetic acid salts, hydrofluoric acid and hydrofluoric acid salts have similar effects, but for the same reason, acetic acid / T-Cr (weight ratio) is 0.1 to 1.0 and hydrofluoric acid / hydrofluoric acid is respectively. T-Cr (weight ratio) is limited to the range of 0.01 to 0.1.

The drying temperature is at least 150 ° C. in order to reduce the amount of Cr ^{6+ in} the chromate film after treatment so that the Cr ⁶⁺ / T-Cr (weight ratio) is 0.2 or less. However, if the temperature exceeds 300 ° C., the continuity of the film is impaired and the corrosion resistance deteriorates, so the range is limited to 150 ° C. to 300 ° C.

The amount of the chromate film deposited is 10 on a dry weight basis.
~ 150 mg / m ² . This is because if it is less than 10 mg / m ² , the corrosion resistance becomes insufficient, and if it exceeds 150 mg / m ² , the color tone of the chromate becomes too strong and the color tone of the galvanized steel sheet is impaired.

[0020]

EXAMPLES The present invention will be specifically described below based on examples.

[Examples] The present invention will be specifically described below based on Examples. (Example) one side of the weight per unit area of 60 g / m ² galvannealed steel sheet (Fe alloying ratio of 10%), one side of the basis weight is 60 g / m ²
Hot-dip galvanized steel sheet and 5% Al-galvanized steel sheet (one-sided basis weight 60 g / m ² ) as raw materials, chromic anhydride, distilled water, ethyl alcohol, phosphoric acid and / or phosphoric acid salt, acetic acid and / or Alternatively, each of the chromate treatment liquids having the composition shown in Table 1 containing acetate, hydrofluoric acid and / or hydrofluoric acid salt was applied by a bar coater, and the heating temperature of the steel sheet was changed to 50 to 350 ° C by the sheet temperature. , The corrosion resistance of each steel sheet,
The amount of residual hexavalent chromium, black rust resistance, and blackening were evaluated, and the results are shown in Table 2. The evaluation method is as follows.

Corrosion resistance is determined by a salt spray test (JIS Z 23
The evaluation was made by the 5% white rust generation time in 71).

The amount of residual hexavalent chromium was determined by immersing the chromate-treated steel sheet in boiling water for 30 minutes and measuring the amount of chromium adhering to the steel sheet before and after that with fluorescent X-rays. The eluted chromium was expressed as hexavalent chromium in percentage. .

Black rust resistance and blackening resistance are 50 × 100.
After stacking two 2 mm test pieces in a vinyl bag and keeping them in a thermo-hygrostat at a temperature of 60 ° C and a relative humidity of 95% or more for 2 weeks, black rust was generated on the superposed surfaces. , Blackening was observed and evaluated.

Ten sets of each test piece were prepared. For galvannealing, the rate of occurrence of black rust (10 sets, percentage of black rust in 20 sheets) was 5% with hot-dip galvanized steel sheet.
Al galvanized steel sheet is blackened (difference in L value before and after the test by SM color computer (manufactured by Suga Test Instruments) ΔL = L
₁ (before the test) -L ₂ (after the test) was evaluated).

As is clear from the above results, the alloyed galvanized steel sheet of the present invention example No. 1 to
All 8 have excellent black rust resistance.

On the other hand, Comparative Example No. 1 having a low drying temperature.
No. 9 had a high residual hexavalent chromium content of 29% and a black rust occurrence rate of 8
It is as high as 0%. In addition, Comparative Example N having a high drying temperature of 330 ° C.
o. No. 10 has sufficient blackening resistance, but is inferior in corrosion resistance. Comparative Example N containing neither phosphoric acid, acetic acid, nor hydrofluoric acid
o. In No. 11, unevenness is generated and black rust is also partially generated. Comparative Example No. In Nos. 12 to 14, since phosphoric acid, acetic acid, and hydrofluoric acid were in excess, the etching was too strong, causing unevenness and partial black rust. Comparative Example No. 1
Since No. 5 has a low reduction rate, residual hexavalent chromium exceeds 20% and black rust is generated. Comparative Example No. In No. 16, the reduction rate is too high and unevenness occurs.

For hot-dip galvanized steel sheets, No. 17-1
As shown in FIG. 9, those within the range of the present invention exhibit excellent blackening resistance (ΔL is 3 or less). (No black rust is observed in hot dip zinc and 5% Al-Zn alloy plating.)

On the other hand, in Comparative Example No. In No. 20, the drying temperature was too high and there was not sufficient corrosion resistance. No. 21 is too low and the blackening resistance is not sufficient. Comparative example No. 22
In Nos. 24 to 24, since phosphate, acetate and hydrofluoric acid are excessive, unevenness occurs and blackening resistance is slightly inferior.

In the case of the 5% Al-Zn alloy plated steel sheet of the present invention example No. As shown in Nos. 25 to 27, those within the range of the present invention exhibit excellent blackening resistance ([Delta] L10 or less, 5% Al-Zn alloy has an original L value as high as about 80.
ΔL is larger than that of molten Zn which is about 0). On the other hand, Comparative Example No. 28, when the drying temperature is too high, the corrosion resistance deteriorates, and Comparative Example No. If it is too low as 29, the blackening resistance is poor.

Further, in Comparative Example No. When the reduction ratio is too high as in No. 30, uneven chromate treatment occurs, and Comparative Example N
o. When it is too low as shown by 31, the blackening resistance is poor. From the above, it is apparent that the present invention can produce a chromate-treated zinc or zinc-based plated steel sheet excellent in black rust resistance and blackening resistance.

[0032]

[Table 1]

[Table 2]

[0033]

[Table 3]

[Table 4]

[0034]

EFFECTS OF THE INVENTION According to the present invention, the chromate-treated hot-dip galvanized steel sheet (alloy which is excellent in black rust resistance and blackening resistance without any post-treatment by controlling the composition of the chromate treatment liquid and the drying temperature) (Including hot-dip galvanized steel sheet and hot-dip 5% Al-Zn plated steel sheet) can be manufactured.

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[Procedure amendment]

[Submission date] February 28, 1992

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] Claim 2

[Correction method] Change

[Correction content]

[Procedure Amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0021

[Correction method] Change

[Correction content]

(Example) An alloyed zinc-plated steel sheet having a basis weight of 60 g / m ² (Fe alloying ratio of 10%), a hot-dip galvanized steel sheet having a basis weight of 60 g / m ^{2 on} one side, and 5
% A1-Galvanized steel sheet (one-sided basis weight 60 g / m ² ).
As a raw material, chromic anhydride, distilled water, ethyl alcohol, phosphoric acid and / or phosphate, acetic acid and / or acetate, hydrofluoric acid and / or hydrofluoric acid were added to obtain the compositions shown in Table 1. The chromate treatment liquid was applied with a bar coater, and the heating temperature of the steel sheet was changed to 50 to 350 ° C. at the sheet temperature to evaluate the corrosion resistance, the amount of residual hexavalent chromium, the black rust resistance, and the blackening property of each steel sheet. The results are shown in Table 2. The evaluation method is as follows.

[Procedure 3]

[Document name to be amended] Statement

[Name of item to be corrected] 0032

[Correction method] Change

[Correction content]

[0032]

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Sachiko Suzuki             1 Kawasaki-cho, Chiba-shi, Chiba Kawasaki Steel Co., Ltd.             Corporate Technology Research Division (72) Inventor Takao Kurisu             1 Kawasaki-cho, Chiba-shi, Chiba Kawasaki Steel Co., Ltd.             Corporate Technology Research Division

Claims (2)

[Claims] 1. Trivalent chromium / hexavalent chromium (weight ratio) is 1/4 to 1 / on the surface of a zinc or zinc alloy plated steel sheet.
An aqueous solution containing 2 to 20% by weight of chromic acid and a chromic acid reducing product in the range of 1 as metal chromium,
One or more selected from phosphoric acid, acetic acid, hydrofluoric acid and salts thereof are converted into phosphoric acid, acetic acid and hydrofluoric acid in terms of phosphoric acid / total chromium amount (weight ratio) of 0.05. / 1
~ 2.0 / 1, acetic acid / total chromium amount (weight ratio) 0.1 /
1 to 1.0 / 1, 0.0 in hydrofluoric acid / total chromium amount (weight ratio)
After applying an aqueous solution containing in the range of 1/1 to 0.1 / 1, this is heated and dried in the temperature range of 150 ° C. to 300 ° C., and the hexavalent chromium / total chromium amount (weight ratio) is 0.20. A method for producing a chromate-treated zinc- or zinc-based alloy-plated steel sheet having excellent black rust resistance, which comprises forming a chromate-treated film having a chromium deposition amount of 10 to 150 mg / m ² below. 2. The chromate treatment excellent in black rust resistance according to claim 1, wherein the zinc or zinc alloy plated steel sheet is a hot dip galvanized steel sheet alloyed hot dip galvanized steel sheet or a hot dip Zn-Al alloy plated steel sheet. A method for producing a zinc- or zinc-based alloy-plated steel sheet.